The present invention pertains generally to devices and methods for separating and segregating the constituents of a multi-constituent material. More particularly, the present invention pertains to devices for efficiently initiating and maintaining a multi-species plasma in one portion of a chamber and then separating the ions in the multi-species plasma according to their respective mass to charge ratios in a second portion of the chamber. The present invention is particularly, but not exclusively, useful as a high-throughput filter to separate the high-mass particles from the low-mass particles in a plasma chamber having two, axially opposed plasma injectors.
There are many reasons why it may be desirable to separate and segregate a multi-constituent material into its separate constituents. One such application where it may be desirable to separate a multi-constituent material is in the treatment and disposal of hazardous waste. For example, it is well known that of the entire volume of nuclear waste, only a small amount of the waste consists of radionuclides that cause the waste to be hazardous. Thus, if the radionuclides can somehow be separated from the non-hazardous ingredients of the nuclear waste, the handling and disposal of the radioactive components can be greatly simplified and the associated costs reduced.
Indeed, many different types of devices, which rely on different physical phenomena, have been proposed to separate mixed materials. For example, settling tanks which rely on gravitational forces to remove suspended particles from a solution and thereby segregate the particles are well known and are commonly used in many applications. As another example, centrifuges which rely on centrifugal forces to separate substances of different densities are also well known and widely used. In addition to these more commonly known methods and devices for separating materials from each other, there are also devices which are specifically designed to handle special materials. A plasma centrifuge is an example of such a device.
As is well known, a plasma centrifuge is a device which generates centrifugal forces to separate charged particles in a plasma from each other. For its operation, a plasma centrifuge necessarily establishes a rotational motion for the plasma about a central axis. A plasma centrifuge also relies on the fact that charged particles (ions) in the plasma will collide with each other during this rotation. The result of these collisions is that the relatively high-mass ions in the plasma will tend to collect at the periphery of the centrifuge. On the other hand, these collisions will generally exclude the lower mass ions from the peripheral area of the centrifuge. The consequent separation of high-mass ions from the relatively lower mass ions during the operation of a plasma centrifuge, however, may not be as complete as is operationally desired, or required.
Apart from a centrifuge operation, it is well known that the orbital motions of charged particles (ions) in a magnetic field, or in crossed electric and magnetic fields, will differ from each other according to their respective mass to charge ratio. Thus, when the probability of ion collision is significantly reduced, the possibility for improved separation of the particles due to their orbital mechanics is increased. For example, U.S. Pat. No. 6,096,220, which issued on Aug. 1, 2000 to Ohkawa, for an invention entitled xe2x80x9cPlasma Mass Filterxe2x80x9d and which is assigned to the same assignee as the present invention, discloses a device which relies on the different, predictable, orbital motions of charged particles in crossed electric and magnetic fields in a chamber to separate the charged particles from each other. In the filter disclosed in Ohkawa ""220, the magnetic field is oriented axially, the electric field is oriented radially and outwardly from the axis, and both the magnetic field and the electric field are substantially uniform both azimuthally and axially. As further disclosed in Ohkawa ""220, this configuration of fields causes ions having relatively low-mass to charge ratios to be confined inside the chamber during their transit of the chamber. On the other hand, ions having relatively high-mass to charge ratios are not so confined. Instead, these larger mass ions are collected inside the chamber before completing their transit through the chamber. The demarcation between high-mass particles and low-mass particles is a cut-off mass Mc which is established by setting the magnitude of the magnetic field strength, B0, the positive voltage along the longitudinal axis, Vaxis, and the radius of the cylindrical chamber, xe2x80x9caxe2x80x9d. Mc for this configuration can then be determined with the expression:
Mc=ea2(B0)2/8Vaxis.
In the filter disclosed in Ohkawa ""220, a multi-species plasma is introduced into one end of a cylindrical chamber for interaction with the crossed electric and magnetic fields. As further disclosed in Ohkawa ""220, the fields can be configured to cause ions having relatively high-mass to charge ratios to be placed on unconfined orbits. These ions are directed toward the cylindrical wall for collection. On the other hand, ions having relatively low-mass to charge ratios are placed on confined orbits inside the chamber. These ions transit through the chamber toward the ends of the chamber. It can happen, however, that some low-mass ions, as they undergo separation, are directed toward the end where the multi-species plasma is being introduced into the chamber. This allows the low-mass ions to be re-mixed with the multi-species plasma, lowering the separation efficiency of the plasma mass filter.
One way to overcome the end loss described above is to use a tandem plasma mass filter. Specifically, U.S. Pat. No. 6,235,202, which issued on May 22, 2001 to Ohkawa, for an invention entitled xe2x80x9cTandem Plasma Mass Filterxe2x80x9d and which is assigned to the same assignee as the present invention, discloses a device wherein the feed material is introduced midway between the ends of a cylindrical plasma chamber. After separation in the plasma chamber, the light ions are collected at both ends of the cylindrical chamber. Because a plasma needs to be created near the center of the plasma chamber, the tandem mass filter requires a high density vapor jet or some other injector to introduce vapor into the chamber. Once the vapor is introduced into the chamber, an r-f antenna or some other mechanism is required to heat and ionize the vapor. The present invention reduces the end loss problem in a different way than the tandem plasma mass filter. Specifically, the present invention contemplates maintaining a multi-species plasma in one portion of a plasma chamber and then separating the ions in the multi-species plasma according to their respective mass to charge ratios in a second portion of the chamber. Because of the location of the second portion of the chamber and the configuration of the crossed electric and magnetic fields, the ions are not directed toward the first portion of the chamber during separation, and there is little re-mixing of separated ions.
In light of the above, it is an object of the present invention to provide devices for efficiently initiating and maintaining a multi-species plasma in one portion of a plasma chamber and then separating the ions in the multi-species plasma according to their respective mass to charge ratios in a second portion of the chamber. It is another object of the present invention to provide an efficient, high-throughput filter to separate the high-mass particles from the low-mass particles with little or no re-mixing of separated ions. It is yet another object of the present invention to provide a filter to separate the high-mass particles from the low-mass particles in a plasma chamber that accommodates two, axially opposed plasma injectors. Yet another object of the present invention is to provide devices and methods for separating and segregating the constituents of a multi-constituent material which are easy to use, relatively simple to implement, and comparatively cost effective.
In overview, the present invention is directed to devices and methods for separating and segregating the constituents of a multi-constituent material. In particular, for the operation of the present invention, a multi-species plasma is first created from the multi-constituent material and introduced into a first portion of a plasma chamber using two, axially opposed plasma injectors. Once the multi-species plasma is established in the first portion, ions in the plasma diffuse into a second portion of the plasma chamber where the ions are separated according to their respective mass to charge ratios by their interaction with crossed electric and magnetic fields.
In greater detail, the device in accordance with the present invention includes a chamber having a substantially cylindrical wall that extends between a first end of the chamber and a second end of the chamber. The cylindrical wall is centered on a longitudinal axis. Primary magnetic coils are selectively arranged on the outside of the chamber wall and are activated to generate a substantially uniform magnetic field, B0, inside the chamber that is oriented substantially parallel to the longitudinal axis.
An injector is provided at each end of the plasma chamber to create a multi-species plasma from the multi-constituent material and inject the multi-species plasma into the plasma chamber. Each injector includes a first section for evaporating the multi-constituent material and a second section for heating and ionizing the resulting vapors. The ionization and heating creates a multi-species plasma having ions of relatively high-mass to charge ratio (M1) and ions of relatively low-mass to charge ratio (M2). In greater structural detail, the second section of the injector includes a substantially cylindrical wall having a first end for receiving vapors and a second end for emitting a plasma jet. Preferably, a radio-frequency (rf) antenna is provided to heat and ionize vapors in the second section of the injector. Importantly, the diameter of the cylindrical injector wall is smaller than the diameter of the cylindrical wall of the plasma chamber.
For the present invention, the injectors are positioned at the ends of the plasma chamber with the cylindrical walls of the injectors centered on the longitudinal axis of the plasma chamber. With this cooperation of structure, the plasma jets emitted by the injectors are directed along the longitudinal axis of the plasma chamber. In greater detail, the opposed injectors establish and maintain a multi-species plasma in a core portion of the plasma chamber. The core portion is a substantially cylindrical volume, centered on the longitudinal axis of the plasma chamber and extending from the first end of the plasma chamber to the second end of the plasma chamber. In size, the core portion has an approximate diameter equal to the diameter of the cylindrical walls of the injectors.
Within the plasma chamber, the core portion is surrounded by an annular volume that extends from the core portion to the cylindrical wall of the plasma chamber. During operation of the present invention, ions of the multi-species plasma diffuse radially from the core portion into the annular volume where they are separated according to their respective mass to charge ratios using crossed electric and magnetic fields. As indicated above, an axially aligned magnetic field, B0, is established inside the plasma chamber (in both the core portion and the annular volume) by the primary coils. Additionally, the device includes one or more primary electrodes for creating a radially oriented electric field in the annular volume portion of the plasma chamber. Specifically, the primary electrode(s) are positioned at the end(s) of the plasma chamber between the wall of the injector and the wall of the plasma chamber. With this cooperation of structure, the primary electrode(s) establish a positive voltage (Vctr) at the cylindrical boundary between the core portion and the annular volume, and a substantially zero potential at the wall of the chamber. Importantly, the primary electrodes create little or no electric field within the core portion of the plasma chamber.
During operation of the present invention, ions from the plasma that is established in the core portion of the plasma chamber diffuse into the annular volume. Once the ions reach the annular volume, they are separated according to their respective mass to charge ratio by the crossed electric and magnetic fields. Specifically, in the crossed fields, an ion having a relatively low-mass to charge ratio (M2) is confined inside the chamber during its transit of the chamber. As such, the low-mass ions (M2) move toward one of the ends of the chamber and strike one of the primary electrodes for collection. On the other hand, in the crossed fields, an ion having a relatively high-mass to charge ratio (M1) is not so confined. Instead, these larger mass ions strike a collector mounted on the inside of the chamber wall before completing their transit through the chamber. Specifically, for a chamber wall that has a radius xe2x80x9caxe2x80x9d and a core portion that has a radius xe2x80x9cdxe2x80x9d, ions having a mass (M1) that is greater than a cut-off mass, Mc (M1 greater than Mc) will be collected at the chamber wall, where
Mc=eB02(a2xe2x88x92d2)/8Vctr.
Here xe2x80x9cexe2x80x9d is the ion charge. Ions having a mass (M2) that is less than a cut-off mass, Mc (M2 less than Mc) will transit through the chamber and be collected at the primary electrodes.
A number of modifications can be made to the device described above to increase the rate at which the ions diffuse from the core portion to the annular portion of the plasma chamber (i.e. the ion loss rate). By increasing the ion loss rate, the overall throughput of the device can be increased. One way to increase the ion loss rate from the core portion is to apply a small radial electric field within the core portion using one or more secondary electrodes. The resulting friction force between rotating ions and neutrals will cause ion drift in the radial direction. As detailed further below, the magnitude of this radial electric field must be limited to prevent ion separation from occurring within the core portion. In another modification to increase the ion loss rate, secondary coils are provided to create a magnetic mirror at each end of the cylindrical core portion. As detailed further below, these magnetic mirrors create a plasma instability in the core portion that increases the rate at which the ions diffuse from the core portion to the annular volume.